Field of the Invention
The present invention relates to a vibration actuator suitable for use in a magnetic field environment, which brings a vibration element and a driven element into pressure contact with each other and excites vibration in the vibration element to thereby move the vibration element and the driven element relative to each other, and a medical system provided with the vibration actuator.
Description of the Related Art
In recent years, research and development of a medical assistant robot for performing medical practice on a subject (patient) while acquiring a diagnostic image of the subject by using a magnetic resonance imaging diagnostic apparatus (hereinafter referred to as the “MRI diagnostic apparatus”) have been actively performed. Further, an open-type MRI diagnostic apparatus provided with a gantry having a large opening or a gantry having a large space in the center thereof has come into wide-spread use, and there is an increasing possibility of intervention of a medical assistant robot or a doctor into the MRI diagnosis.
The static magnetic field generated by the MRI diagnostic apparatus has so large a magnitude as approximately 1.5 [T] to 3.0 [T] and is very strong. Further, in the MRI diagnostic apparatus, to determine three-dimensional position information which is used in acquiring a diagnostic image of a subject, with high accuracy, the magnetic field accuracy is controlled with very high accuracy, and a gradient magnetic field that varies with time in three axial directions is generated. Therefore, when a conductive material is used for a member that forms a closed loop in any of a medical assistant robot and other medical instruments, which is brought into the vicinity or the inside of the gantry of the MRI diagnostic apparatus, it is required to prevent the Lorentz force from being generated by a variable magnetic field and the conductive material from adversely affecting the variable magnetic field.
As an actuator used in a variable magnetic field environment which is controlled with high accuracy, there has been proposed a vibration actuator using an electromechanical energy conversion element which is typically formed e.g. of piezoelectric ceramics. Differently from an electromagnetic motor, the vibration actuator frictionally drives a driven element using vibration excited in a vibration element. Therefore, the vibration actuator has features that it generates a high thrust or torque in a low-speed region, with high responsiveness, and is capable of directly driving the driven element without using mechanical transmission means, such as a gear or a belt. The vibration actuator also has an advantage that a holding force or a holding torque is generated by friction between the vibration element and the driven element in a power-off state, and hence there is no need to use braking means, such as a brake.
In a type of the vibration actuator, a vibration element is formed by joining a piezoelectric element to an elastic body made of metal or the like, and AC voltages, which are different in phase, are applied to the piezoelectric element to thereby excite vibration in the vibration element in a specific vibration mode. In such a vibration actuator, an elliptic motion is generated on a surface of the elastic body which is in contact with the driven element, whereby the vibration element and the driven element are rotationally or linearly moved relative to each other.
As an example of the vibration actuator, there has been disclosed in Japanese Patent Laid-Open Publication No. 2007-159211 a vibration actuator that uses a vibration element having an annular shape, and an elastic body made of a metallic material having a high resonance sharpness (e.g. a steel material, such as stainless steel) is used in the annular vibration element. To cause the piezoelectric element to generate flexural vibration, such as bending vibration, it is necessary to generate a potential difference in a piezoelectric body (piezoelectric ceramics) as a component of the piezoelectric element. For this reason, in Japanese Patent Laid-Open Publication No. 2007-159211, a GND (ground) portion of the piezoelectric element and the elastic body are electrically connected to each other by a conductive junction made of a conductive material, such as solder, to ground the elastic body, whereby connection of the piezoelectric element to the GND potential is realized by making use of conductivity of the elastic body.
However, in the case where the vibration actuator including such a vibration element formed by using a metallic material having a high resonance sharpness as described in Japanese Patent Laid-Open Publication No. 2007-159211 is installed within or in the vicinity of the gantry of the MRI diagnostic apparatus, there are brought about, for example, three problems described below.
Firstly, disturbance may be caused in a magnetic field of the MRI diagnostic apparatus, which is controlled with high accuracy. More specifically, in a case where a member made of a conductive material includes a portion which has an annular shape, forming a closed loop, variable current flowing through the closed loop generates a new variable magnetic field by an induced electromotive force generated due to temporal variation in magnetic flux penetrating this closed loop. Therefore, the member having a closed loop portion, made of a conductive material, may disturb a magnetic field of the MRI diagnostic apparatus, which is necessary for encoding of spatial coordinates and is controlled with high accuracy.
Secondly, noise may be superimposed on a diagnostic image. More specifically, in a case where a member made of a conductive material includes a closed loop portion, an induced electromotive force is generated due to temporal variation in all magnetic fluxes penetrating the closed loop according to the Maxwell-Ampere law. Thus generated induced electromotive force generates electromagnetic waves due to variable current flowing through the closed loop, and these electromagnetic waves may be superimposed on various signals as electromagnetic noise. Therefore, the member having a closed loop portion made of a conductive material can be a source of noise for the MRI diagnostic apparatus and the peripheral devices.
Thirdly, unnecessary mechanical vibrations may be generated. More specifically, in a case where a member made of a conductive material has a closed loop portion, if a magnetic flux penetrating the closed loop temporally varies, the above-mentioned induced electromotive force causes a temporally varying current to flow through the closed loop portion. As a result, assuming that a current vector is represented by I and a magnetic flux vector by B, the Lorentz force F which temporally varies is applied to the closed loop in a direction of a vector product of I×B, which may cause unnecessary mechanical vibrations. Therefore, there is a possibility that the member having a closed loop portion made of a conductive material adversely affects the performances of the vibration actuator and the medical assistant robot.
To avoid these problems, it is envisaged to use a non-magnetic dielectric (insulating) material, such as engineering ceramics, engineering plastics, or a composite material (e.g. FRP (fibre-reinforced plastic)), for the component members of the vibration actuator, including the driven element and the elastic body. However, for example, in a case where the elastic body is dielectric, the GND portion of the piezoelectric element cannot be grounded via the elastic body as described in Japanese Patent Laid-Open Publication No. 2007-159211.